Apparatus and method for determining conductivity of wiring installations

ABSTRACT

An apparatus and method for determining the conductivity of wiring installations of power distribution circuits is described. Selected portions of the circuit are subjected to alternating and direct current potentials of predetermined magnitude from distinct sources. The resultant currents are alternately indicated on distinct meters at a rate higher than the mechanical inertia thereof, permitting substantially simultaneous visual observation of both indications.

United States Patent Florance et a1.

[ Mar. 21, 1972 APPARATUS AND METHOD FOR DETERMINING CONDUCTIVITY OFWIRING INsTALLATIoNs Inventors: Douglas A. Florence, 1110 HillsideDrive, Vestal, NY. 13850; Lloyd P. Nordholm,

215 Castle Creek Road, Binghamton,

Filedz' Nov. 24, 1969 Appl. No.: 879,108

u.s. Cl ..324/5l, 324/62, 324/73 Int. cI....- .con 27/00, G0lr 31/02Field of Search ..324/51, 54, 5s, 73, 62, 66

References Cited UNITED STATES PATENTS Swartwout 324/73 Brown et a1..324/62 X Primary Examiner-Gerard R. Strecker An0rneyFrederick E.Bartholy [57] ABSTRACT An apparatus and method for determining theconductivity of wiring installations of power distribution circuits isdescribed. Selected portions of the circuit are subjected to alternatingand direct current potentials of predetermined magnitude from distinctsources. The resultant currents are alternately indicated on distinctmeters at a rate higher than the mechanical inertia thereof, permittingsubstantially simultaneous visual observation of both indications.

5 Claims, 3 Drawing Figures PATENTEDMARZ] I972 FIG. l

TIMER FIG.2

DC DC Ac Ac n-AC n OFF , TIME FIG. 3.

INVENTURS A. FLORANCE LLOYD P. NORDHOLM DOUGLAS ATTORNEY APPARATUS ANDMETHOD FOR DETERMINING CONDUCTIVITY OF WIRING INSTALLATIONS BACKGROUNDOF THE INVENTION dards set up by the National Electrical Code.Heretofore, such testing methods involved the use of ohmmeters toindicate the continuity and resistance of selected portions of theinstallation. Resistance checks alone are not sufficient for powerinstallations which, at the present time, in practically all cases usealternating current. Nor does the resistance determination alone fulfillthe requirements of the National Electrical Code of which Article 1lO-IO, partially quoted states:

The overcurrent protective devices, the total impedance and othercharacteristics of the circuit to be protected shall be so selected andcoordinated as to permit the circuit protective devices used to clear afault without the occurrence of extensive damage to the electricalcomponents of the circuit.

Article 250-51, entitled Effective Grounding," reads as follows:

The path to ground from circuits, equipment,

and conductor enclosures shall 1) be permanent and continuous and (2)shall have ample carrying capacity to conduct safely any currents liableto be imposed on it, and (3) shall have impedance sufficiently low tolimit the potential above ground and to facilitate the operation of overcurrent devices in the circuit. As seen, the code specifies, in bothinstances, impedance," i.e., the resistance to alternating current. Thismay be quite different from that of the ohmic resistance of the wiresand conduits. The following tabulation, taken from the booklet GroundingElectrical Distribution Systems for Safety" by Eustace Soares, showsimpedance with respect to the resistance of standard conduit sizes.

SUMMARY OF THE INVENTION It is seen from the above that, due to theferromagnetic properties of the material, the impedance may at times befive times greater than the ohmic resistance. It is, therefore, of greatimportance in the testing of electrical installations that theconductivity be determined not only to direct current, but toalternating current as well, in order to correlate the electricalcharacteristics of such installations.

A primary object of this invention is to provide an apparatus and methodfor testing electrical power installations which will indicate the trueconductivity of any portion so tested.

It is a further object of this invention to provide an apparatus fortesting selected portions of wiring installations by means ofalternating current and direct current whereby the impedance as well asthe resistance of the selected portions may be determined.

It is a particular feature of the invention that the apparatus fortesting the conductivity of electrical installations indicates suchconductivity to alternating as well as to direct current, utilizingindividual indicating instruments switched alternately in rapidsuccession to the portions to be tested, allowing simultaneousobservation of both indications.

It is a particular advantage of the invention that the apparatusperforming the testing function is simple in construction and requiresno particular aptitude for its application and operation.

A further feature of the invention resides in the method of operation,entailing simple steps which may be performed with ease by anyonewithout requiring professional skill.

Other objects, features and advantages will be apparent from thefollowing description of the invention, pointed out in particularity inthe appended claims, and taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic circuit of the testing apparatus;

FIG. 2 is a schematic circuit, showing a timer for the actuation of theapparatus; and 7 FIG. 3 is a curve showing the sequential switching ofthe circuits correlated to time.

DETAILED DESCRIPTION Referring to the Figures, the schematic circuit ofthe testing apparatus in accordance with the invention comprises analternating current power source represented by a variable voltagetransformer 11, the input terminals of which may be connected to asuitable power line indicated as an alternating current source 12. Thevariable output of the transformer 11 between rider l3 and commonconductor 14 connects to the primary winding 15 of a stepdowntransformer 16. The output of the secondary winding 17 connects to thecircuit of the first indicating meter 20. This circuit, utilizing acertain type of meter, generally comprises a shunt resistor 21 and avariable series resistor 22 connected to the terminals of the meter 20.A parallel resistor 25 completes this circuit. The function of thisresistor will be explained later. Suffice it ,0 say that application ofvoltage to the primary winding 15 will result in a certain current flowin the closed circuit, comprising the secondary winding 17, andresistors 21 and 25. This current will be indicated by the meter 20 interms of voltage drop across resistor 21. The value of the currentdepends of course upon the position of the rider 13 of the transformer11.

The test circuit is connected in shunt with the resistor 25 andcomprises the common conductor 26 to which is connected on the testleads 27. The other test lead connects to the moving contact 29 of therelay 30. The normally closed stationary contact 31 connects throughconductor 32 to the junction point 33 of resistors 21 and 25. From thisis seen that when the relay 30 is not energized, the test leads 27 and28 are placed in parallel with resistor 25, across which the testvoltage is developed for the circuit to be tested.

For the testing of the direct current properties of a circuit there isprovided a direct current source 35, shown here, by way of example, as abattery. One terminal of the battery 35 connects to the common conductor26 and thereby to the test lead 27. The other terminal connects to thedirect current meter circuit comprising variable resistor 36 and shuntresistor 37. One terminal of resistor 37 connects by means of conductor38 to the normally open stationary contact 39 of the relay 30 and theother terminal of resistor 37 to another stationary contact 40 of therelay 30. The corresponding moving contact 41 thereof connects to oneterminal of capacitor 42, the other terminal thereof being connected toconductor 38. One terminal of the direct current meter 45 connects toconductor 38 and the other terminal to the normally closed stationarycontact 46 of the relay 30.

Examining the direct current meter circuit, it is seen that in the restposition, when relay 30 is not energized, the meter terminals are placedacross the capacitor 42 so that the meter would indicate a charge ofthis capacitor. This indication may be in terms of voltage or in otherdesired magnitudes, such as ohms, by providing a suitable meter scale.On the other hand, when the relay 30 is energized so that contacts 40and 41 are closed, the capacitor 42 is placed across resistor 37 to becharged by the voltage drop thereacross, due to current flow.

The actuation of the relay 30 iseffected from a suitable source,indicated here by terminals 50 and 51 of conductors.

52 and 52. These terminals may of course be connected to any sourcesuitable for the operation of the relay 30. This may be either direct oralternating current. In the case of alternating current,. theseterminals may be connected to the source 12; and, in the case of directcurrent, to a suitable rectifier filter assembly also supplied from thesource 12.

A timer, shown in block diagram, represents a device which is capable ofperiodically supplying current to the output thereof represented byconductors 53 and 54 to which the winding of relay 30 is connected. Inone position, the transfer switch 55 in this circuit connects the timerto the relay 30, and, in the other, disconnects the timer and connectsthe relay to conductor 52. As mentioned before, the function of thetimer is simply to switch the relay 30 to the supply source for aselected predetermined time and switch it ofi for another selected time.As such, it may have various conventional forms, such as amotor-operated switch or, if desired, an electronic switch, thesequential operation of which may depend upon an oscillator producingtimed pulses of desired duration. The timer may be disconnected from thesupply source by means of the switch 56 and the relay 30 may beconnected to the supply source by actuation of the transfer switch 55,thereby bypassing the timer.

Prior to describing the operation of the testing apparatus, it is to benoted that it is intended, as mentioned before, for the testing ofwiring installations of electrical power circuits. In suchinstallations, a point-to-point check of desired portions is ratherimportant to make certain that conductivity is accurately determined atspecified currents. By the term conductivity" is meant, thedetermination of reactance and resistance and, particularly, thecomparison or the ratio between alternating current resistance, namely,impedance and ohmic resistance. In such installations the totalimpedance of the various branches must of course be extremely small inthe range of fractions of an ohm. This is one reason why ordinaryresistance checks by an ohmmeter are inadequate, inasmuch as theimpedance to alternating current may be relatively high; whereas thedirect current resistance is sufficiently low to'indicate a satisfactoryconductive path. In order to measure fractions of an ohm, adequatecurrent must be caused to flowv in the test portion to produce a voltagedrop thereacross and in any series-connected element, which can bereadily indicated. The circuit of meter 20 provides this current flow byvirtue of the adjustable voltage input to the primary winding 15 oftransformer 16. The low voltage, high current output thereof enables theuser to cause a relatively large current flow in the low impedance testcircuit, i.e., between the conducting path to which test leads 27 and 28are connected. The internal resistance of the meter circuit isdetermined by the shunt resistor 25. The latter is chosen to be in thefractional ohm range so as not to exceed to a great extent the lowimpedance of the test portion. The adjustment of the reading of themeter 20, namely, the scale indication, by means of variable resistor22, permits selection of the most useful scale on the meter forindicating the additional current which will flow through the test leadsand through the portion of the circuit to be tested. Moreover, since theapplication is alternating current, the meter will indicate theconductivity of the test portion to this current. The application oflarge currents to the test circuit has another important aspect, namely,to ascertain the capability of the circuit to carry large currents,i.e., the ampere capacity thereof, referred to in technical parlance asthe ampacity.

Aside from impedance measurements, in such installations it is alsoimportant to determine the ohmic resistance of the test portion. Thispurpose is served by the meter METHOD OF OPERATION In the operation, theinput terminals of transformer 11 are connected to the power lineavailable at the side where the wiring installation is to be tested. Itis, of course, understood that during such tests the installation itselfis disconnected from the power line source.

At the start of the test, the timer is disconnected so that relay 30 isnot energized. The rider 13 of the transformer 11 is placed in aposition which will result in a desired current flow in the resistor 25.The voltage produced by this current flow will be the test voltageapplied to the portion of the installation to be tested. The indicationof the meter 20 is then adjusted by variable resistor 22 to a point onthe scale which is considered most suitable for the indication of thechange of current which will result due to the connection of the testleads 27 and 28 to the circuit to be tested. It should be pointed outthat resistor 25 is of a known value and so is the resistance of eachtest lead. The latter may vary depending upon the length which mustnecessarily be used and the size of the test lead conductor chosen,bearing in mind that these must be able to carry the current which mayflow in the test circuit. In practice, the

test current may have a value between 25 to several hundred amperes,depending upon the current-producing capabilities of the transformer 16and the type of installation to be tested. The test leads 27 and 28 areplaced between a selected portion of the wiring installation. This may,for example, be the length of a conduit which encases conductors of theinstallation in order to determine the ferromagnetic properties or toany other portion which will have to carry the normal operating current.

The change in indication of the meter 20 will represent the current flowin the circuit to be tested and will be higher than at the start.Although such indication would normally be shown as a current change andwould have to be calculated in terms of impedance, it is just' asfeasible to have the meter scale calibrated in ohms directly.

In order to evaluate the ohmic resistance of the test portion, the relay30 must be energized to transfer the test lead 28 by means of theclosure of contacts 29 and 39 to the direct current side of theapparatus. This may be effected by placing the switch 55 in suchposition as to make contact with terminals 50, bypassing the timer.Aside from closure of contacts 29 and 39, when the relay 30 isenergized, contacts 40 and 41 are also closed so that capacitor 42 isnow placed across resistor 37. Current from battery 35 will now flowthrough the test portion between test leads 27 and 28 and resistor 37and variable resistor 36. The voltage drop across resistor 37 producedby this current will now charge capacitor 42. When the relay 30 isdeenergized, test leads 27 and 28 are transferred to the alternatingcurrent portion of the apparatus and condenser 42 transferred across theterminals of the meter 45. The latter will thus indicate the chargereceived by condenser 42 at the time it was placed in parallel withresistor 37. The charge of condenser 42 is of course dependent upon thevoltage drop across resistor 37 produced by the current flow and thusmay be used as an indication of the resistance of the portion of thecircuit tested. The scale indication of meter 45 may also be chosen byadjustment of variable resistor 36 toa desired position at the start,for example, by short circuiting test leads 27 and 28. i

In order to have a simultaneity of indication, the timer is placed inoperation. At certain intervals it will energize the relay 30 andthereby transfer the test circuit from the alternating current side tothe direct current side of the apparatus. The time intervals are sochosen that proper charging ofthe capacitor 42 be effected during theactuation of relay 30 and adequate time be provided for measuring thecharge of the condenser during deenergizing of relay 30 while theportion of the circuit under test is subjected to alternating currentflow. The frequency of switching of relay 30, i.e., the number of timesit is energized during a given period is so chosen as to be fast enoughto take advantage of the kinetic inertia of the meter movements. In apractical application it was found that an elf period for the relay 30of nine-tenths of a second and an on period of about one-tenth of asecond gave satisfactory results.

The switching cycle of the timer is illustrated in the curve of FIG. 2,wherein the horizontal line represents time interval in seconds and thevertical line, the on-off state of the timer, i.e., the current flow inthe winding of relay 30. It is to be understood that this curve is forthe purpose of illustration of the desired condition and does notnecessarily represent the shape of the actual current flow in thewinding of relay 30. The shape of the curve of the actual current wouldof course be influenced by the inherent inductance and capacitance ofthe relay. However, this does not enter into the effective operationthereof. The recurrent and sequential operation of the moving contacts29 and 41 of the relay 30 will thus switch the test circuit back andforth from the alternating current side to the direct current side insuch rapid succession that the meter indications will appearsubstantially stationary so that both can be observed simultaneously bythe operator. In this manner, a direct and simultaneous indication ofthe alternating current and direct current flow in the test portion maybe taken and evaluated as to the ratio of impedance to resistance. Thisratio is particularly indicative of the condition of the wiringinstallation to be tested.

The direct current portion need not utilize large currents inasmuch aspurely ohmic resistance is of importance. On the other hand, thealternating current portion has the capability of applying largecurrents to the test circuits in an order larger than would normally becarried. This is particularly advantageous in determining thesuitability of conductor joints as well as the proper grounding of suchinstallations.

If it is desired to obtain greater differentials in the reactance overresistance ratio of the test portion, higher than line frequency may beutilized for the alternating current evaluation. For this purpose, afrequency converter of the rotary or solid state type may be interposedbetween the power source 12 and the transformer 1 1.

The invention in its broader aspects is not limited to the specificembodiments herein shown and described but changes may be made withinthe scope of the accompanying claims without departing from theprinciples of the invention and without sacrificing its chiefadvantages.

What is claimed is:

1. Apparatus for determining the conductivity of the wiring installationof electric power circuits comprising a first current indicating meterand circuit therefor, a power source of alternating current supplyingsaid circuit, a second current indicating meter and circuit therefor, apower source of direct current supplying said last mentioned circuit, apair of test leads adapted to be clamped between desired points of saidwiring installation, switching means for alternately and sequentiallyconnecting said leads to said meter circuits, thereby indicating currentflow between said test points in reference to alternating and directcurrent due to the applied potentials from said respective sources saidswitching means including a relay for transferring said test leadsbetween said meter circuits, a power source for said relay, and a timercontrolling the sequential energizing of said relay, and the duration ofthe time during which said relay is actuated, whereby the indications ofsaid meters due to the switching rate of said switching means permitapparent simultaneous observation of both alternating and directcurrents.

2. Apparatus in accordance with claim 1 wherein said direct currentmeter circuit includes a capacitance, said switching means beingoperable during connection of said test leads to said meter circuit forbridging said capacitance across a predetermined resistance carrying thetest current from said direct current source and transferring saidcapacitance during disconnection of said test leads between'theterminals of said indicating meter.

3. Apparatus in accordance with claim 1 wherein each of said meters hasa face plate and a movement actuating a pointer, said switching meanshaving a transfer rate higher than the inertia of said movements wherebythe indications of said meters remain substantially stationary foralternate observation.

4. The method of testing the conductivity of the wiring installation ofpower circuits which comprises, alternately and sequentially applying toselected portions of said circuits an electrical potential ofalternating current of predetermined magnitude, and an electricalpotential of direct current of predetermined magnitude, indicating theresultant currents from said applied potentials in sequential order at arate dependent upon the inertia of the indicating means and permittingapparent simultaneous observation of said currents.

5. The method of testing the conductivity of wiring installation ofpower circuits which comprises, subjecting in succession a selected testportion of said installation to an alternating current source for afirst predetermined time duration, causing a current flow and indicatingthe magnitude of said current in terms of alternating currentconductivity; transferring said test portion to a direct current sourcefor a second predetermined time duration, thereby causing a directcurrent flow in said portion charging a capacitance to a voltageproportional to said direct current flow during such time; thereaftertransferring said test portion back to said alternating current sourcefor the duration of said first time and simultaneously indicating thevoltage charge of said capacitance in terms of direct currentconductivity during said first time duration,

lOlOZS 0747

1. Apparatus for determining the conductivity of the wiring installationof electric power circuits comprising a first current indicating meterand circuit therefor, a power source of alternating current supplyingsaid circuit, a second current indicating meter and circuit therefor, apower source of direct current supplying said last mentioned circuit, apair of test leads adapted to be clamped between desired points of saidwiring installation, switching means for alternately and sequentiallyconnecting said leads to said meter circuits, thereby indicating currentflow between said test points in reference to alternating and directcurrent due to the applied potentials from said respective sources saidswitching means including a relay for transferring said test leadsbetween said meter circuits, a power source for said relay, and a timercontrolling the sequential energizing of said relay, and the duration ofthe time during which said relay is actuated, whereby the indications ofsaid meters due to the switching rate of said switching means permitapparent simultaneous observation of both alternating and directcurrents.
 2. Apparatus in accordance with claim 1 wherein said directcurrent meter circuit includes a capacitance, said switching means beingoperable during connection of said test leads to said meter circuit forbridging said capacitance across a predetermined resistance carrying thetest current from said direct current source and transferring saidcapacitance during disconnection of said test leads between theterminals of said indicating meter.
 3. Apparatus in accordance withclaim 1 wherein each of said meters has a face plate and a movementactuating a pointer, said switching means having a transfer rate higherthan the inertia of said movements whereby the indications of saidmeters remain substantially stationary for alternate observation.
 4. Themethod of testing the conductivity of the wiring installation of powercircuits which comprises, alternately and sequentially applying toselected portions of said circuits an electrical potential ofalternating current of predetermined magnitude, and an electricalpotential of direct current of predetermined magnitude, indicating theresultant currents from said applied potentials in sequential order at arate dependent upon the inertia of the indicating means and permittingapparent simultaneous observation of said currents.
 5. The method oftesting the conductivity of wiring installation of power circuits whichcomprises, subjecting in succession a selected test portion of saidinstallation to an alternating current source for a first predeterminedtime duration, causing a current flow and indicating the magnitude ofsaid current in terms of alternating current conductivity; transferringsaid test portion to a direct current source for a second predeterminedtime duration, thereby causing a direct current flow in said portioncharging a capacitance to a voltage proportional to said direct currentflow during such time; thereafter transferring said test portion back tosaid alternating current source for the duration of said first time andsimultaneously indicating the voltage charge of said capacitance interms of direct current conductivity during said first time duration.